1. Field of the Invention
The present invention relates to color photography and more particularly, it relates to a novel class of two-equivalent magenta couplers which are suitable for use in silver halide color photographic light-sensitive materials.
2. Description of the Prior Art
It is known that, by color development of a silver halide color photographic material, a color developing agent of aromatic primary amine type is oxidized and reacts with a coupler to form a dye, such as indophenols, indoanilines, indamines, azomethines, phenoxazines, phenazines and the like, thus forming a color image. In this type, the subtractive color process is ordinarily used for color reproduction and yellow, magenta and cyan color images are formed, which are, respectively, the complementary colors of blue, green and red. For example, a coupler of the acylacetanilide or dibenzoylmethane type is used for forming a yellow color image, a coupler of the pyrazolone, cyanoacetophenone or indazolone type is used for forming a magenta color image and a coupler of the phenol type, such as a phenol and a naphthol, is used for forming a cyan color image.
In one of the most preferred embodiments of color photographic light-sensitive materials, the dye image forming couplers are incorporated into a silver halide emulsion. These couplers which are incorporated into the emulsion must be non-diffusible (diffusion resistant) in the binder matrix of the emulsion.
The color image-forming couplers of the prior art are almost all four-equivalent couplers which require theoretically four moles of silver halide as an oxidizing agent for forming one mole of the dye through the coupling reaction. On the contrary, a two-equivalent coupler having an active methylene group which is substituted with a group capable of being split off through the coupling of an oxidized product of an aromatic primary amine developing agent requires only the development of two moles of silver halide for forming one mole of the dye. Since the quantity of silver halide required for forming a dye with a two-equivalent coupler is one half of that required with an ordinary four-equivalent coupler, the two-equivalent coupler had many advantages in that a thinner light-sensitive layer can be used and the layer can be processed rapidly. In addition, the photographic properties and economy can increased through a reduction in the layer thickness.
Several attempts have been made to convert 5-pyrazolone type couplers which have been conventionally used as a magenta color forming coupler to two-equivalent couplers. For example, the 4-position of the pyrazolone is substituted with a thiocyano group as described in U.S. Pat. Nos. 3,214,437 and 3,253,924, an acyloxy group as described in U.S. Pat. No. 3,311,476, an acyloxy group as described in U.S. Pat. No. 3,419,391 a 2-triazolyl group as described in U.S. Pat. No. 3,617,291, a halogen atom as described in U.S. Pat. No. 3,522,052, and the like.
However, when these 4-position substituted pyrazolone couplers are employed, some disadvantages occur in that a remarkable color fog is produced, in that the coupling reactivity is insufficient, in that the couplers per se are chemically unstable and change into compounds which can not form dyes or in that many difficulties are encountered in the preparation thereof.
It is also known that the 4-position of the 5-pyrazolones can be substituted with an alkylthio group, an arylthio group or a heterocyclic thio group as described in U.S. Pat. No. 3,227,554. However, most these known thio-substituted pyrazolone compounds have the disadvantages that their reactivity with the oxidation products of an aromatic primary amine color developing agent is not suitable, that the mercapto compounds which are formed upon the coupling reaction have a very severe photographic effect when used in a conventional color photographic light-sensitive material, and that the couplers are chemically unstable.